Disclosed herein are methods for purification of aromatic polyethers, and more particularly to methods for purification of aromatic polyetherimides and polyethersulfones.
Various types of aromatic polyethers, particularly polyetherimides, polyethersulfones, polyetherketones, and polyetheretherketones have become important as engineering resins by reason of their excellent properties. These polymers are typically prepared by the reaction of salts of dihydroxyaromatic compounds, such as bisphenol A (BPA) disodium salt, with dinitroaromatic molecules or dihaloaromatic molecules. Examples of suitable dihaloaromatic molecules include bis(4-fluorophenyl)sulfone, bis(4-chlorophenyl)sulfone, and the analogous ketones and bisimides as illustrated by 1,3-bis[N-(4-chlorophthalimido)]benzene.
According to U.S. Pat. No. 5,229,482, the preparation of aromatic polyethers by displacement polymerization may be conducted in the presence of a relatively non-polar solvent, using a phase transfer catalyst which is substantially stable under the temperature conditions employed. Suitable catalysts include ionic species such as guanidinium salts. Suitable solvents disclosed therein include o-dichlorobenzene, dichlorotoluene, 1,2,4-trichlorobenzene, and diphenyl sulfone.
It is desirable to isolate aromatic polyether from a reaction mixture free from contaminating species that may affect the polymer's final properties in typical applications. In a typical halide displacement polymerization process, contaminating species often include alkali metal halide and other alkali metal salts, residual monomer species, and residual catalyst species. For maximum efficiency of operation it is desirable to recover any solvent employed and other valuable compounds, such as catalyst species, and to provide waste streams which do not contaminate the environment. In particular it is often desirable to recover alkali metal halide, especially sodium chloride, for recycle to a brine plant for production of sodium hydroxide and chlorine.
Many conventional techniques are used to purify polymer-containing organic solutions. For instance, extraction with water and settling by gravity in a mixer/settling tank has been used for removal of aqueous-soluble species. However, water extraction methods are problematic when the water phase emulsifies with or does not phase separate efficiently from the organic phase. For example, polyethers in anisole solutions present difficulties when mixing with water and separating by settling. Pure water typically will not separate from anisole or polymer/anisole solutions after mixing near room temperature because the difference between water and anisole densities is very small. Under these conditions emulsions may form. Even if the first stage of extraction is performed under conditions where density differences drive the separation of water (for example, treatment with brine), the final stage of extraction of water-soluble species still requires use of relatively pure water, which is more prone to emulsification. The end result is that even though alkali metal halide and/or catalyst may be transferred to the aqueous phase, the entrained water in the emulsified organic phase prevents a high recovery of both the halide and the catalyst. High purity polymer solutions with minimal residual species may then be extremely difficult if not impossible to obtain. Another constraint is that the time for separation of the aqueous and organic phases must be fast, preferably on the order of minutes, so that separation rates do not slow down production. A method is needed that more efficiently removes alkali metal halide and other alkali metal salts, residual monomer species, and residual catalyst species.